In typical vehicle air conditioners the air passage contains a heater core supplied by engine coolant as well as an evaporator which has refrigerant supplied by an engine-driven compressor. Incoming air is cooled by the evaporator when the compressor is operating, and the air is heated if it is directed through the heater core. A temperature door controlled by an actuator is positioned in the air passage to block air flow through the heater core, or to direct all or some of the air through the heater core depending on the desired air temperature. When the temperature door is fully closed in the fullcold position, the airflow can still "scrub off" some unwanted heat from the backside of the heater core. This unwanted heat reduces the efficiency of the cooling system. Radiation and conduction from the heater core are also significant contributors to unwanted passenger compartment heating. A remedy for these heating conditions is to shut off the coolant flow to the heater core so that, once the heater core cools down, the air leakage, radiation, or conduction do not add heat to the cold air stream.
Previously such coolant valves were operated by an electrical or a vacuum switch in the control head responsive to temperature lever position and coupled by wire or vacuum line to the coolant shutoff valve, by a cable extending from the temperature lever to the valve, or by a microswitch mounted on the temperature door and coupled to the shutoff valve. Each of these special items adds complexity and cost to the vehicle, and in some cases are not practical to employ. These costs are particularly exacerbated in the case of a rear air conditioning system with its own control head in addition to a front system with its own control head, both requiring their own shutoff valves. It is desirable then to dispense with special switches, cables and the like while maintaining the coolant shutoff function.